inet6_opt_init.3: Some enhancements
- Be consistent with RFC references, so add a space after 'RFC'
- Add a LIBRARY section
- Use standard integer types in the SYNOPSIS section
Obtained from: DragonflyBSD
Differential Revision: https://reviews.freebsd.org/D27548
(cherry picked from commit 211ceb62e85d8d01ea89af0de8e43430de550e72)